Hmm. Although it is original enough, I can't help but feel you are only taking away from normal gameplay. No building, no basekilling, no real incentive for the human team to attack... seems to me this would be highly likely to turn into a massive campathon, with all the humans huddling together in the window room or similar, chainguns and mass drivers pointed at the door, ready to fill anything that may enter with as much lead as they can.
To be honest, I can't imagine the ammo store lasting much longer than ten minutes, then the helpless blaster gang will last maybe another five. In the meantime, there may be a lot more waiting than playing.